Abstract: The present invention relates to a viscosity modifier for a thermoplastic polyester resin comprising 100 parts by weight of polymer particles prepared by suspension polymerization having a glass transition temperature of at least 60° C. and volume average particle size of 50 to 500 ?m, which are coated with 0.5 to 30 parts by weight of an emulsion polymer prepared by emulsion polymerization, wherein the polymer particles prepared by suspension polymerization have reactivity with thermoplastic polyester resin, a thermoplastic polyester resin composition and a molded article.